Process of preparing a polyethylenepolyisoolefin blend



stretched return only slightly States F PREPARING A POLYETHYLENE- POLYISOOLEFIN BLEND to Esso Research and Engineering Company, a corpo ration of Delaware PROCESS No Drawing. Application October 20, 1955 Serial No. 541,836

Claims. (Cl. 260-455) This invention is concerned with the preservation of food in thin, flexible and impervious casings and relates particularly to an improved material for'the formation of such casings.

Although this invention is applicable to any food product in the preservation of which the use of thin, flexible and impervious casings is desirable and is applicable to preservation at any temperature, it is particularly suitable for use in preserving meats by refrigeration or low temperature treatment.

It has been proposed to enclose foodstuffs to be preserved by freezing or chilling in thin-walled bags of flexible materials, such as rubber or a rubber hydrohalide, and to evacuate the air from the bag and seal it hermetically. Very quick freezing may then be obtained because the covered carcass may be brought into direct contact with the refrigeration medium, such as air or brine, without sufiering either contamination or desiccation. Meats stored in such bags after freezing have shown an improved quality over those not so protected.

Casing materials lacked to a serious degree various of the propertiesdesired. If ordinary rubber is used, the vulcanizing agents the food. Un-

may impart obnoxious odors or tastes to vulcanized rubber bags must be coated with soaps or dusted with talc, starch or an equivalent material to reduce their stickiness so that they can be shipped and stored and so that the completed packages can behandled conveniently and stored without adhering to eachother. Unless these materials are removed from the inside of the bags before the food products are inserted they may leave an objectionable coating on the food. It is, of course, an expensive and commercially impracticable procedure to remove the material from the inside of the bag without removing it from the outside at the same time. Starch, recognized as the most satisfactory of these materials, tends to promote the growth of molds.

The bags heretofore proposed are subject to puncturing and tearing in handling, even at low temperatures. This defect has necessitated the use of bags having relatively thick walls, adding to the expense and'interfering with the heat transfer through the bags when the products are frozen.

Many of the advantages of using envelopes in food preservation result from the decreased evaporation and decreased possibility of contamination that an impervious wrapping secures. Rubber materials heretofore proposed lack the high degree of moisture irnpermeability desired in envelopes in which food products are likely to be stored for long periods of time.

Envelopes made from such materials exhibit a high degree of permanent set after stretching, particularly at low extrusion temperatures, that is, the envelopes, when to their original size. In this condition they are unstable to a rise in temperature, springing back when'heated to a size about-25% smaller than the original size. 7 In packaging articles in the bags, advantage may be taken of this property of heretofore proposed, however, have i.

atent 7 vinyl aromatic.

- 2,854,435.. Patented Sept. 30, 1958..

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2 the material to shrink the bag into intimate contact with the article by subjecting the bag momentarily to a temperature of about 212 F. or higher.

Bags of other materials which have been proposed are defective either because they require "after application heat treatment at undesirably high temperatures or because of the high cost of fabricating thematerial into a completed food covering or because of a low degree of pliability and deformability. For example, solid polyethylene, having a molecular weight in the range of 10,000 to 100,000, or more, preferably 15,000" to 25,000, such as may be made by'known methods, e.g., polymerization under high pressure and temperature" by the use of a small'amount of oxygen o'r'other agent as promoter, has been found to have'many valuable characteristics for this purpose but it requires" temperatures of 215 to 230 F. in order to obtain adequate shrinking. However, since the simplest method of heating"tl ie'poly mer is by dipping in hot water, it is necessary to add a high boiling liquid such as propylene glycol to the water to increase its boiling point. This raises problems of getting rid of the glycol and is therefore urides'irable' It is accordingly the principal aim and object or the invention to provide an inexpensive "envelope, suitable for packaging food products, which is non-tacky, pliable and deformable, impermeable to moisture and both thin and strong. A further object is to provide an envelope which possesses to a high degree the mechanical properties desired in food casings and which at the same time is transparent and attractive in appearance and will form a saleable commercial package. A still further object of the invention is to provide an envelope which may be shrunk down onto the product by a simple method to cause the covering to cling to a product such as beef as if it were the skin of the animal, and to produce covers which in their shrunken condition are transparent and relatively free from wrinkles and folds.

In accordance with the present invention it has now been discovered that the peculiar physical characteristics which are necessary to adapt polyethylene to the covering of foods and particularly to the preservation of meats are obtained by a special blending of about 25-40%, preferably about 30% of a polyiso-olefin having a molecular weight above 140,000 with the polyethylene. If desired the polyiso-olefin may contain about 10% of a low molecular weight copolymer of an iso-olefin and a A particularly suitable composition is one composed of 70% polyethylene, 27% polyisobutylene of 140,000-300,000 molecular weight, 3% of a copolymer of isobutylene and styrene containing 55 to ,6S,.% styrene and having an intrinsic viscosity below 0.5.

The polyiso-olefin used in this invention is prepared by polymerizing an iso-olefin of 4 to 5 carbon atoms per molecule, preferably isobutylene, by a low temperature procedure and in the'presence of a solution of a-Friedel- Crafts type catalyst in a low-freezing solvent which does not form a complex therewith such as methyl or ethyl chloride or carbon di-sulfide. The temperatures applied range from about -40 C. to 'l00 C. or -l50 C. or lower, the preferred range being from C. to -103 C. This polymer has an average molecular weight ranging between 100,000 and 500,000 Staudinger.

The iso-olefin-vinyl aromatic hydrocarbon copolymer may be made broadly within the'teachings' of Patent 2,274,749 but for the purposes of this invention, it must be made under certain restricted combinations of conditions; namely, by copolymerizing 40 to-.70% by weight, preferably 55 to 65% by weight of a polymerizable monoolefinic aromatic compound with an alkene of 3 to 8, preferably 4 to 5 carbon atoms, at a temperature between about -l0 C. and l00 C. Temperaturesbetwe'en -l0 C. and -50 C., preferably between -'-18" -C. and

C. are suitable for low molecular weight material having intrinsic viscosities below 0.5. Temperatures below C., preferably below 70 C., are suitable for high molecular weight polymers having intrinsic viscosity A lower alkyl halide liquid is used as solvent for the reaction mixture. The catalyst may appropriately be a solution of aluminum chloride in a lower alkyl halide such as methyl chloride or ethyl chloride. Other known Friedel-Crafts catalysts, preferably predissolved in a solvent, may be used. A copolymer of isobutylene and styrene containing to styrene and having an intrinsic viscosity below 0.5 is particularly suitable.

The various polymers must be blended in just the correct manner in order to obtain a mixture which will form expanded films capable of being shrunk at 212 F. This is accomplished, according to this invention by preparing and aminor proportion of polyethylene, the softer polyethylene beingadded to the tougher isobutylene. A portion of the blend is then removed and replaced with a sufficient amount of polyethylene to give the desired final blend. -The blend is then formed into an expanded envelope by tubular extrusion at 250 F.

At the food packing plant, after the animal is slaughtered and properly cleaned and prepared, the carcass is placed in one of the expanded rubber envelopes. The open end of the bag, i. e., the neck portion of the envelope, is twisted and securely tied to form an air tight 4 down against the carcass so tightly that it appears almost as a transparent skin and hardly any spot is left on complicated cuts where the envelope is not in close contact with the meat.

After the envelope has been shrunk into contact with the meat by heating, the meat may merely go to a chill room and be chilled according to conventional practice, but, in all cases to secure the best results from the invention, the envelope should be left on the meat until it must be cut up for sale by the retail butcher.

The advantages of the invention will be better understood from a consideration of the following experimental data which are given for the sake of illustration, but without the intention of limiting the invention thereto.

Example I Films were prepared from 100% polyethylene; mixtures of polyethylene, 30% polyisobutylene of 200,000 molecular weight; mixtures of 70% polyethylene and 30% polyisobutylene of 300,000 molecular weight; 70% polyethylene, 27% polyisobutylene of 200,000 molecular weight and 3% of a copolymer of isobutylene and styrene containing 60% styrene and having an intrinsic viscosity less than 0.5; and a similar composition containing 27% isobutylene of 300,000 molecular weight. These compositions were extruded at 250 F. into tubular film, the film was then formed into bags and shrunk at 212 F. The following data were obtained:

Polyethylene, 21,000 mol. wt

Polylsobutylene, 300,000 mol. wt: I

Polyisobutylene, 200,000 mol. wt copolymer of styrene (I. V. 0.5)

Total Banbury Timej'ididfiiilj degrees Dump Temperature,

Mixing Procedure (see below) Tensiles, p. s. i.* Elongation, percent Blend Homogeneity.

Percent shrinking 212 styrene and isobut A B C D E 1,530 800 1,150 950 1,350 1.350 600 360 525 530 680 700 good poor poor good good good 1.0 1.0 1.0 13. 5 6. 4 15.1

Made on molded test pads. (A) Banbury hotPolyethyle parable to the following condltio (B) Banbury hot-Polyisobuty mixed for the remainder of th ne pellets added to hanbury to give simular work to polymer so as to be comns.

lene homogenized for 2 minutes in the banbury. Polyethylene pellets added e total time.

(0) Banbury hot-Polylsobutylene homogenized for 2 minutes in the banbury. Polyethylene pellets added and mixed for the remainder of seal. A vacuum line is attached to a small tube formed on the envelope, and the air which may be trapped between the carcass and the bag is sucked out. At this moment, the bag, which very loosely enfolds the carcass, is pushed against it by the outside atmospheric pressure, forming a great number of wrinkles and folds. Alternatively, instead of exhausting the air by vacuum, it may be pressed out from between the carcass and the bag by means of external pressure.

The polymer blend in an expanded or extended condition is physically unstable to a rise in temperature. At room temperature it will contract toward its original size very slowly or not at all, but, if the temperature be raised approximately to ZOO-212 F. it springs back to about of its original size almost instantaneously. Therefore the wrapped carcass is heated very briefly in order to cause the restoring forces in the rubber bag to become active and to cause the folds and wrinkles to disappear. This may be done by dipping the carcass for no more than a few seconds into boiling or near boiling water or the carcass may be pushed quickly through a hot room at about 200-212 F. on its way to the cooler. Only this momentary heating is necessary to shrink the coating i of between about 200 F. and

the total tim Banbury hot-Polyisobutylene homogenized for removed and polyethylene pellets added minutes. 30% of the Banburied for 15 minutes. The with additional polyethylene to make a final blend of isobutylene-styrene copolymer.

The above data clearly show that polyethylene shrinks only about 1.0% at 212 F. and that blends of polyisobutylene and polyethylene likewise shrink only slightly at 212 F. unless they are prepared in accordance with the blending technique of this invention, when up to about 15% shrinkage occurs.

The nature of the present invention having been thus secured by Letters Patent is:

1. A process for blending polyisobutylene having a molecular weight of at least about 140,000 with polyethylene which comprises homogenizing the polyisobutylene, removing about 30% of the homogenized polyisobutylene and replacing it with an equivalent amount of polyethylene, homogenizing the mixture, removing a sufficient amount of homogenized mixture ethylene and about 30% polyisobutylene, and mixing the resulting blend until it is capable of shrinking at least about 6% to 25% when heated to a temperature level about 212 F.

2. A process for preparing a blend of about 60 to 75 wt. percent of polyethylene having a molecular weight between about 10,000 and about 100,000 and about to wt. percent of a polyisoolefin having a molecular weight of between about 140,000 and about 300,000, which comprises mixing the polyisoolefin until substantially homogeneous, removing a substantial portion up to about 30 wt. percent of the polyisoolefin and replacing it with an equivalent amount of polyethylene, homogenizing the mixture formed, removing a suflicient amount of the resulting homogenized mixture and replacing it with an equal amount of polyethylene so that the resulting mixture contains about to wt. percent of polyethylene and about 25 to 40 wt. percent of the polyisoolefin, and homogenizing the mixture formed until it is capable of shrinking at least about 6 to 15% when heated to a temperature level of between about 200 F. and about 3. Process according to claim 2 in which the polyisoolefin is polyisobutylene.

4. Process according to claim 2 ethylene has a molecular weight and 25,000.

5. Process according to claim 2 in which the polyisoolefin is initially blended with up to about 10 wt. percent of a copolymer of isobutylene and styrene before the addition of polyethylene.

in which the polybetween about 15,000

References Cited in the file of this patent UNITED STATES PATENTS 2,339,958 Sparks Jan. 25, 1944 2,369,471 Latham Feb. 13, 1945 2,686,725 Cornwell Aug.- 17, 1954 2,699,396 Francis Jan. 11, 1955 OTHER REFERENCES Modern Packaging, pages -102, March 1955. 

1. A PROCESS FOR BLENDING POLYISOBUTYLENE HAVING A MOLECULAR WEIGHT OF AT LEAST ABOUT 140,000 WITH POLYETHYLENE WHICH COMPRISES HOMOGENIZING THE POLYISOBUTYLENE, REMOVING ABOUT 30% OF THE HOMOGENIZED POLYISOBUTYLENE AND REPLACING IT WITH AN EQUIVALENT AMOUNT OF POLYETHYLENE, HOMOGENIZING THE MIXTURE, REMOVING A SUFFICIENT AMOUNT OF THE RESULTING HOMOGENIZED MIXTURE AND REPLACING IT WITH AN EQUAL AMOUNT OF POLYETHYLENE SO THAT THE RESULTING MIXTURE CONTAINS ABOUT 70% POLYETHYLENE AND ABOUT 30% POLYISOBUTYLENE, AND MIXING THE RESULTING BLEND UNTIL IT IS CAPABLE OF SHRINKING AT LEAST ABOUT 6% TO 25% WHEN HEATED TO A TEMPERATURE LEVEL OF BETWEEN ABOUT 200*F. AND ABOUT 212*F. 